This invention relates to wheel speed sensors and particularly to wheel speed sensors for vehicle anti-skid systems.
Devices for sensing vehicle wheel speeds, and particularly changes in wheel speeds are important components in braking anti-skid or skid control systems in use today. In such anti-skid systems, these sensing devices generate signals which are carried to an electronic controller adapted to control a brake pressure modulation valve. Because the frequency of the signal from the sensing device is proportional to wheel speed, a properly programmed controller can modulate brake pressure so as to reduce or preclude wheel skids or locks when the vehicle is being braked.
Most anti-skid sensing devices operate to generate electrical signals created by a changing magnetic flux field, where a rotor or exciter rotates with the rotating portions of the wheel relative to a nonrotating stator or pick-up device. "Outboard" sensing devices (i.e., devices generally located axially outwardly of the wheel) normally include a rotor and stator type arrangement, while "inboard" sensing devices (i.e., devices generally located axially inwardly of the wheel) normally include an exciter and pick-up type arrangement.
In outboard sensing devices, a toothed or notched ferromagnetic rotor, operatively connected to a rotary portion of the wheel assembly, is disposed adjacent a toothed or notched ferromagnetic stator, operatively connected to a nonrotating portion, such as the axle. Proper disposition of a permanent magnet and coil relative to the rotor and stator enables variable frequency currents or signals to be sent to a controller.
In inboard sensing devices, an exciter having a specially constructed or contoured annular surface portion is operatively connected to a rotatable structure of the wheel and brake assembly, such as a wheel hub or rotatable brake component. The exciter is positioned to rotate past a pick-up device or "sensor" operatively connected to a nonrotable structure of the wheel and brake assembly such as an axle or nonrotatable brake component. The sensor, which comprises a magnet surrounded by a coil, senses changing magnetic field of flux caused by the adjacent rotation of the specially designed surface of the exciter. The changing flux field induces a variable signal in the sensor coil which is thereupon carried to the controller.
Sensor components of inboard type sensing devices are generally of two types, "bipolar" or "unipolar". Bipolar sensors usually feature two spaced prongs or flanges which straddle the rotatable exciter, such that the exciter rotates within the gap between the spaced prongs or flanges. Unipolar sensors usually feature a single pole portion or surface which is in nearly direct contact with the exciter. The bipolar type sensor is relatively insensitive to gap fluctuations caused by any relative movements between it and the exciter. Unipolar sensors, however, are sensitive to relative movement, or changes in position between it and the exciter.
Because of the sensitivity of unipolar type sensors, the manner in which such sensors are mounted is important. Such sensors must be mounted to prevent the sensor from undergoing any axial and/or rotational movement relative to the nonrotatable portion of the wheel assembly to which it is secured once the sensor has been properly positioned adjacent the exciter.